X enogenic graft n: placement of a tissue obtained from an animal species over, or into, an affected area of another species. Also known as a heterologous graft. xeroradiography n: the dry photoelectric process by which an X-ray image is captured using metal plates coated with a semiconductor, such as selenium. xerostomia n: dryness of the mouth due to lack of or inadequate secretions (eg, inadequate salivary secretions). Y east n: classified in the kingdom Fungi, these eukaryotic microorganisms are mostly unicellular but can become multicellular through the formation of pseudohyphae or strings of con- nected budding cells. Yeast can reproduce by mitosis, asymmetric division. or budding. Yeast can become pathogenic in the body, including the oral cavity, espe- 74 cially in people who are immunocompromised through disease or drug therapy. Young's modulus n: describes the rigidity or stiffness of a material usually given the symbol E. It is a measure of elasticity equal to the ratio of the stress acting on a substance to the strain produced. A higher modulus (GPa, psi) signifies a greater rigidity or stiffness to the material. Also called modulus of elasticity. Z ero-degree teeth n: posterior denture teeth having flat planes or zero-degree cusp angles relative to the horizontal occlusal surface of the tooth. See also nonanatomic teeth. zirconia ceramic post n: a ceramic post used instead of metal posts in the restoration of endodontically treated teeth. The ceramic material is preferred to the metal posts for esthetic reasons. Ceramic posts are usually used with a composite resin or compression ceramic to form the core.